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I'm going to be building a retaining box for a gravel pad using 6x6's - 0.16 lb./cu. ft. MCA pressure treated lumber. The dimensions of the box matched up with available lengths at the lumber yard, so I'm not going to have to cut any of the pieces that I've ordered. Any ends that will be below grade are going to be up against clay soil.

Do I still need to dip/paint the ends with wood preservative, or will the PT chemicals alone be sufficient to prevent rot?

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The pressure treating commonly used today does not penetrate the lumber 100%. I'm assuming mostly financial reasons at play here (time, cost of materials, etc). I highly suggest any cut ends that you see untreated wood at the center, be treated with off the shelf, brush on preservative. You will be well served here. Why take that chance over a few extra minutes, and a relatively small amount of dollars on a presumably several hundred, to several thousand dollar investment?

  • The products I've been finding online (CopperCoat®, Copper-Green®, etc) are not kept in stock at my local hardware stores, so if I'm going to use it, I need to order some to be delivered. Since I'm not making any cuts myself, I wasn't sure if the ends will already have the PT chemicals embedded in them... sounds like it's not a bad idea to add some even if they are already treated though... – CactusCake Jul 10 '17 at 19:47
  • I would recommend ordering some nonetheless. I also suggest brushing it along places where you are putting large fasteners, or holes bored for rebar or similar. Any points of failure where rot tends to take hold can all benefit from a little extra treatment. What's $25 preventative action against $1000 in materials? – BrownRedHawk Jul 10 '17 at 20:02
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I put thin copper metal ( from art supply) over buried and some other ends. This is because I built a deck 20 years ago ( treated 2X6 ) and all of the spots that have gotten bad were ends. When I cut out bad spots I found they were about 3/4" deep. Copper is a strong deterrent to mold, etc ( It is in some of the better preservatives.)

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